1. Field of the Invention
The present invention relates to a method of manufacturing miniaturized devices. In particular, the invention relates to a lithographic method of manufacturing miniaturized devices using a projection exposure system of the liquid immersion type.
2. Brief Description of Related Art
Lithographic processes are commonly used in the manufacture of miniaturized structures such as integrated circuits, liquid crystal elements, micro-patterned structures and micro-mechanical components.
A projection exposure apparatus used for photolithography generally comprises a projection optical system for imaging a patterning structure, commonly referred to as a reticle, onto a substrate, commonly referred to as a wafer. The substrate (wafer) is coated with a photo-sensitive layer, commonly referred to as a resist, which is exposed with the image of the patterning structure using imaging light. The imaging light is generated by an illumination optical system illuminating the patterning structure.
The trend to produce even smaller and more sophisticated miniaturized devices places increasingly high demands on an optical resolution which is achievable with the imaging of the projection exposure system. Conventional projection exposure systems have been designed in view of increasing a numerical aperture of the imaging on a side of the exposed substrate. Increasing the numerical aperture while maintaining the optical performance of the system at a desired level results in that the projection exposure system has an increased number of optical elements with an increased diameter. The efforts to manufacture such projection optical system are significantly increased with increasing numerical aperture.
Recently liquid immersion-type projection exposure systems have been proposed. In such immersion-type projection exposure system, an immersion liquid is disposed in a space between the substrate to be exposed and a front lens of the system disposed closest to the substrate. With such immersion liquid an achievable resolution is significantly higher than in a comparable system having a gas rather than the immersion liquid disposed in the space between the substrate and the front lens.
A design and manufacture of projection optical systems of the immersion-type suffers from a lack of experience with such technology. Among others, a behavior of the immersion liquid is not sufficiently well understood and analyzed at present.
The optical properties of the immersion liquid and in particular its refractive index have to conform with a predefined value used in the design of the system. It is understood that the optical properties of the immersion liquid have to be controlled to a high accuracy during the exposure process.